This invention relates to a method of igniting a subterranean fragmented permeable mass of formation particles containing oil shale within an in situ oil shale retort. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce hydrocarbon-containing liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the hydrocarbon-containing liquid product is called "shale oil."
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is fragmented to form a fragmented permeable mass of formation particles containing oil shale. Retort forming techniques are shown in U.S. Pat. Nos. 4,043,595; 4,043,596; 4,043,597; and 4,043,598, for example. Formation particles at the top of the fragmented mass are ignited to form a combustion zone, and an oxygen-containing gas, such as air, is supplied to the top of the fragmented mass for sustaining the combustion zone and advancing the combustion zone downwardly through the fragmented mass. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to decompose kerogen in the fragmented particles, producing shale oil and gaseous products therefrom in a retorting and vaporization zone. Thus, a retorting zone moves from top to bottom of the fragmented mass in advance of the combustion zone. The shale oil and gaseous products in the retorting zone pass to the bottom of the fragmented mass for collection.
When the fragmented permeable mass of formation particles is formed in the subterranean in situ oil shale retort, the fragmented particles can fill substantially the entire volume so that there is no significant void space above the fragmented mass. Oxygen-containing gas for combustion can be furnished to the fragmented mass by means of holes bored through overlying intact rock. Appreciable difficulty may be encountered, however, in igniting the top of the fragmented mass to support combustion. Ignition requires a substantial amount of heat delivered over a sufficient time to raise the temperature of fragmented particles of oil shale above an ignition temperature of shale oil. Considerable difficulty is encountered in providing burners for such ignition and assuring that ignition has been obtained. In a large retort several burners may be needed at the same time for igniting the top of the fragmented mass at several locations. It is therefore desirable to provide an inexpensive and reliable burner for igniting an in situ oil shale retort from a remote location. It is also desirable to provide a burner suitable for using shale oil for igniting a retort since this is the least expensive fuel available at the retort site.